drivers: clock_control: nxp: Fix building MCG on KW40Z/KW41Z
drivers: clock_control: nxp: Fix building MCG on KW40Z/KW41Z

The KW40Z/KW41Z don't have a Fixed Freq MCG clock so the code
associated with that in get_rate fails to build.  ifdef around
the code with enum kCLOCK_McgFixedFreqClk so things work
correctly.

Signed-off-by: Kumar Gala <383ef5577c6e1178b93f59ec8d0936f76d2a98c4@kernel.org>

zephyrproject-rtos/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,galak/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,galak/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,finikorg/zephyr,finikorg/zephyr